linux-kernel - Re: [RFC PATCH 2/2] init: Allow multi-line output of kernel command line

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Date:   Tue, 19 May 2020 21:58:53 -0700
From:   Joe Perches <joe@...ches.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Chenggang Wang <wangchenggang@...o.com>,
        linux-kernel@...r.kernel.org, Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC PATCH 2/2] init: Allow multi-line output of kernel command
 line

On Wed, 2020-05-20 at 13:41 +0900, Sergey Senozhatsky wrote:
> On (20/05/19 12:42), Joe Perches wrote:
> > +static void __init print_cmdline(char *line)
> > +{
> > +#ifdef CONFIG_PRINTK
> > +	const char *prefix = "Kernel command line";
> > +	size_t len = strlen(line);
> > +
> > +	while (len > PRINTK_LOG_LINE_MAX) {
> > +		char *pos = line;
> > +		char *last_pos = pos + PRINTK_LOG_LINE_MAX - 1;
> > +		char saved_char;
> > +		/* Find last space char within the maximum line length */
> > +		while ((pos = memchr(pos, ' ', len - (pos - line))) &&
> > +		       (pos - line) < PRINTK_LOG_LINE_MAX - 1) {
> 
> Don't you need to also count in the 'prefix' length?

yup.

> > +			last_pos = pos;
> > +		}
> > +		saved_char = line[last_pos - line];
> > +		line[last_pos - line] = 0;
> > +		pr_notice("%s: %s\n", prefix, line);
> > +		prefix = "Kernel command line (continued)";
> > +		line[last_pos - line] = saved_char;
> > +		len -= pos - line;
> > +		line += pos - line;
> > +	}
> > +
> > +	pr_notice("%s: %s\n", prefix, line);
> > +#endif
> > +}
> 
> I like this in general. And I agree that we better handle this
> externally, on the printk() caller side, so that printk() will
> still have sane limits and won't print a 1G string for example.
> 
> I wonder if we need to export PRINTK_LOG_LINE_MAX.

I think a #define works well enough.(

>  Maybe we can
> use here something rather random and much shorter instead. E.g.
> 256 chars. Hmm. How 

	min(some_max like 132/256, PRINTK_LOG_LINE_MAX)

would work.

> many crash/monitoring tools can get confused
> by multiple "Kernel command line" prefixes?

I doubt any as it's an init only function.